The present invention relates to a stratified scavenging two-cycle engine, which purifies exhaust gas with use of catalyst.
Conventionally, in a two-cycle engine, a part of fuel mixture (mixture of fuel and air) supplied into a cylinder is discharged as it is with combustion gas during an exhaust stroke, in its structure. Consequently, unburned gas (mainly containing hydrocarbon HC) is mixed into exhaust gas, which becomes one of the causes of air pollution. As a countermeasure against this, a method of purifying exhaust gas by placing catalyst in an exhaust passage is known.
As a first example thereof, there is an exhaust gas purification device for a two-cycle engine, which is disclosed in, for example, Japanese Patent Laid-open No. 9-268912. According to Japanese Patent Laid-open No. 9-268912, an exhaust passage is connected to an exhaust port open to a cylinder bore, catalyst is placed midway in this exhaust passage, and a variable exhaust timing valve (catalyst protection means) freely opened and closed, which decreases an amount of blow-by gas amount discharged from an exhaust port is provided near the exhaust port. Thereby, the amount of the blow-by gas is reduced by throttling the exhaust port at the time of a low engine speed, exhaust gas is purified, and catalyst is protected.
As a second example, a muffler for an exhaust device of an internal combustion engine, which is disclosed in, for example, Japanese Patent No. 2603033, is known. According to the above Patent, a catalyst converter is provided in the exhaust passage in the muffler, and a reed valve for supplying secondary air blow, which can be opened and closed according to exhaust gas pressure, is provided at a muffler main body in an upstream side of the catalyst converter. Thereby, oxygen concentration in the exhaust gas is increased and purification rate of the catalyst is increased.
However, the above-described prior art has the following disadvantages.
In the exhaust gas purification device disclosed in Japanese Patent Application Laid-open No. 9-268912 of the first example, catalyst is placed in the exhaust passage of an ordinary (specifically, non-stratified scavenging type) two-cycle engine. Consequently, the oxygen concentration in the exhaust gas is low, and the total hydrocarbon purification rate of the catalyst is low. The xe2x80x9ctotal hydrocarbonxe2x80x9d includes hydrocarbon HC and nitric oxide NOx, and this will be abbreviated as xe2x80x9cTHCxe2x80x9d hereinafter. The THC purification rate is expressed by the equation xe2x80x9cTHC purification rate=(THC concentration % before catalystxe2x88x92THC concentration % after catalyst)xc3x97100/THC concentration % before catalystxe2x80x9d. Accordingly, exhaust rate of the THC in the exhaust gas is high in the first example, and it is extremely difficult to meet a requirement of xe2x80x9cTHC exhaust ratexe2x89xa654 [g/kWxc2x7h]xe2x80x9d which is expected to be strongly controlled in near future.
In the muffler for the exhaust device of the internal combustion engine disclosed in Japanese Patent 2603033 as the second example, it is necessary to provide a reed valve, whereby there arise the disadvantages that the number of components increases, and as a result, operations of working the opening and mounting the reed valve are added.
In view of the above-described disadvantages, an object of the present invention is to provide a stratified scavenging two-cycle engine with catalyst, which can meet such a requirement that the THC exhaust rate is 54 [g/kWxc2x7h] or less with use of the catalyst.
In order to attain the above-described object, a first aspect of the stratified scavenging two-cycle engine with catalyst according to the present invention has a structure in which in a stratified scavenging two-cycle engine including a piston, a cylinder in which the piston is housed slidably up and down, a crankcase connected to a lower end portion of the cylinder and having a crank chamber therein, an exhaust hole and a scavenging hole which are formed in a side wall of the cylinder, a scavenging passage for connecting the scavenging hole and the crank chamber, an air feed passage connected to the scavenging passage and feeding air via a check-valve, and a mixture feed passage for supplying mixture, which is supplied with fuel from fuel feed means, to the crank chamber, and in which an air feed volume ratio R of an air feed volume flowing through the air feed passage to a mixture feed volume flowing through the mixture feed passage is within a range of 0.7xe2x89xa6Rxe2x89xa61.4 during an intake stroke in which the crank chamber becomes negative pressure, oxidation catalyst is placed midway in an exhaust passage in a downstream of the exhaust hole.
According to the above structure, during the intake stroke in which the crank chamber of the stratified scavenging two-cycle engine becomes negative pressure, the air feed volume ratio R (R=qa/Qf) of the air feed volume qa [m3] flowing through the air feed passage to the mixture feed volume of [m3] flowing through the mixture feed passage is within the range of 0.7xe2x89xa6Rxe2x89xa61.4. In this situation, as described in detail in Japanese Patent Laid-open No. 11-107761 proposed by the inventors of the present invention, it is confirmed that the exhaust rate of the THC released from the stratified scavenging two-cycle engine is 67 [g/kWxc2x7h] or less. Since guiding air pushes out exhaust gas in the stratified scavenging two-cycle engine, the oxygen concentration in the exhaust gas becomes higher as compared with a non-stratified scavenging two-cycle engine, and the amount of hydrocarbon becomes one third. The inventors of the present invention confirm that if the oxidation catalyst is placed midway in the exhaust passage provided in the downstream of the exhaust hole of the stratified scavenging two-cycle engine, reheat combustion of hydrocarbon is promoted since the oxygen concentration in the exhaust gas is high, and the purification rate of the catalyst is improved so that the THC exhaust rate becomes 40 [g/kWxc2x7h] or less. Consequently, the target to reduce the THC exhaust rate to 54 [g/kWxc2x7h] or less can be. sufficiently attained. Since the hydrocarbon volume in the exhaust gas becomes one third, the heating value as a result of the reaction of the oxidation catalyst with hydrocarbon reduces, and therefore abnormal rise in temperature is prevented, thus making it possible to improve durability of the oxidation catalyst. Further, rise in temperature of the exhaust gas is reduced. Rise in temperature of the engine itself becomes less, and reduction in durability due to overheating and the like can be prevented. At the same time, heating value of the oxidation catalyst is small, and sufficient purification can be carried out by small-sized catalyst, thus making it possible to reduce the size of the entire engine apparatus. Furthermore, since it is not necessary to provide a reed valve as in the prior arts, the number of components can be reduced and a working operation for the opening for the reed valve becomes unnecessary.
A second aspect of the stratified scavenging two-cycle engine according to the present invention has a structure in which in a stratified scavenging two-cycle engine including a piston, a cylinder in which the piston is housed slidably up and down, a crankcase connected to a lower end portion of the cylinder and having a crank chamber therein, an exhaust hole and a scavenging hole which are formed in a side wall of the cylinder, a scavenging passage for connecting the scavenging hole and the crank chamber, an air feed passage connected to the scavenging passage and feeding air via a check-valve, and a mixture feed passage for supplying mixture supplied with fuel from fuel feed means to the crank chamber, and in which the scavenging passage is formed in the cylinder, or the cylinder and the crankcase, and a scavenging passage volumetric capacity formed by the formed scavenging passage and the check-valve of the air feed passage is 70% or more relative to an air feed volume flowing through the air feed passage during an intake stroke in which the crank chamber becomes negative pressure at the time of full load rated power engine speed, the oxidation catalyst is placed midway in an exhaust passage in a downstream of the exhaust hole.
According to the above structure, the scavenging passage is provided in the cylinder of the stratified scavenging two-cycle engine, or in the cylinder and the crankcase, and the scavenging passage volumetric capacity Vs [m3] formed by this scavenging passage and the check-valve of the air feed passage is 70% or more relative to the air feed volume qa [m3] flowing through the air feed passage during the intake stroke in which the crank chamber becomes negative pressure at the time of the full load rated power engine speed. In this situation, as described in detail in the above-described Japanese Patent Application Laid-open No. 11-107761, it is confirmed that the exhaust rate of the THC discharged from the stratified scavenging two-cycle engine is 54 [g/kWxc2x7h] or less. As described above, the oxygen concentration becomes high and hydrocarbon amount becomes less in the exhaust gas in the stratified scavenging two-cycle engine. As a result, by placing the oxidation catalyst midway in the exhaust passage of the stratified scavenging two-cycle engine as described above, the THC exhaust rate becomes 40 [g/kWxc2x7h] or less, whereby the target to make the THC exhaust rate 54 [g/kWxc2x7h] or less can be sufficiently attained. The inventors of the present invention confirms this fact as a result of the experiments and the like. In addition, by this structure, the durability of the engine and the oxidation catalyst can be improved, and sufficient purification can be performed by small-sized catalyst, and the entire engine apparatus can be made compact. Furthermore, since it is not necessary to provide a reed valve as in the prior arts, the number of the components is small, and the working operation for the opening for the reed valve is not needed.
Further, by combining the above-described first structure and the second structure, more favorable effects can be obtained. As described in detail in Japanese Patent Application Laid-open No. 11-107761, it is confirmed that the exhaust rate of the THC of the stratified scavenging two-cycle engine according to the above-described first and second structure is 67 [g/kWxc2x7h] or less. The present applicant confirms that the THC exhaust rate becomes 40 [g/kWxc2x7h] or less by placing the oxygen catalyst midway in the exhaust passage of the stratified scavenging two-cycle engine, and consequently, the target can be sufficiently attained.